Fluorinated chlorin chromophores for red-light-driven CO2 reduction

The utilization of low-energy photons in light-driven reactions is an effective strategy for improving the efficiency of solar energy conversion. In nature, photosynthetic organisms use chlorophylls to harvest the red portion of sunlight, which ultimately drives the reduction of CO2. However, a molecular system that mimics such function is extremely rare in non-noble-metal catalysis. Here we report a series of synthetic fluorinated chlorins as biomimetic chromophores for CO2 reduction, which catalytically produces CO under both 630 nm and 730 nm light irradiation, with turnover numbers of 1790 and 510, respectively. Under appropriate conditions, the system lasts over 240 h and stays active under 1% concentration of CO2. Mechanistic studies reveal that chlorin and chlorinphlorin are two key intermediates in red-light-driven CO2 reduction, while corresponding porphyrin and bacteriochlorin are much less active forms of chromophores.

Chromophores that have been demonstrated to utilize lowenergy photons for catalytic CO 2 RR are rarely reported in the literature, all of which are based on the most precious metals.A frequent difficulty is that molecules absorbing at long wavelengths often generate insufficient reduction potentials to overcome the large driving forces in CO 2 RR.In 2013, Ishitani and co-workers reported the first molecular system for red-light-driven CO 2 RR using Os−Re supramolecular complexes, achieving a turnover number (TON) of 1138 with >620 nm light 19 .Recently, a photochemical system using a heteroleptic Os(II) chromophore and a Ru(II) catalyst was developed for CO 2 RR to generate HCOOH under irradiation with 725 nm light (TON = 81) 20 .
Furthermore, a Zn porphyrin-sensitized Mn(I) system was demonstrated to reduce CO 2 under 625 nm light, however, its TON CO was lower than 1 21 .
Porphyrin-based compounds have been widely used as blue-light absorbers in artificial photosynthetic systems, either for the reductivehalf [22][23][24][25][26][27][28][29][30] or for the overall reactions [31][32][33] .However, chlorin (Ch), which is a reduced porphyrin adapted by most of the chlorophylls in plants and cyanobacteria (Fig. 1) for harvesting red-light [34][35][36][37] , has not been reported in such a context.The Ch can undergo a 2e − /2H + reduction photochemically to generate a chlorinphlorin (ChPh) [38][39][40][41][42][43] , which exhibits a broad absorption spectrum into the near-infrared region [44][45][46] , as recently determined by Nocera and co-workers 47 .The optical and redox properties of Chs highly suggest that they may serve as active chromophores for red-light-driven CO 2 RR.Here, we present a series of fluorinated Chs (Fig. 1) as chromophores for red-light-driven CO 2 RR in precious-metal-free systems.The structure-function study demonstrates that increasing the number of fluorine substituents on the meso-phenyl group of Ch significantly enhances the activity of CO 2 RR.The systems using a per-fluorinated Ch last over 240 h and give high TONs of 1790 (at 630 nm) and 510 (at 730 nm) in the conversion of CO 2 to CO.

Synthesis and photophysical properties
A library of fluorinated meso-tetraphenyl porphyrins (F x TPP) and chlorins (F x Ch) (x = 0, 4, 12, 20; Fig. 1) were prepared according to procedures described in the "Methods" section.A per-fluorinated bacteriochlorophyll (F 20 BC, Fig. 1) was synthesized by further reduction of F 20 Ch and crystallographically determined (Supplementary Information).In contrast to F x TPP, all F x Ch display strong absorption profiles for red light in N,N-dimethylformamide (DMF) (Supplementary Figs.2-6).Increasing the number of the fluorine substituents on F x Ch shows a general impact on the absorption band at the red-light region, by slightly red-shifting the maximum absorption from 650 to 654 nm, as well as increasing the extinction coefficient (ε) from 3.458 × 10 4 to 5.125 × 10 4 M −1 cm −1 (Table 1).All chlorins exhibit an intense Soret (B) bond (from 405 to 420 nm) and 4 to 5 Q bands (from 504 to 654 nm).As would normally be anticipated for chlorins [48][49][50][51] , red-shift and the higher extinction coefficient for the Q band between 650 and 654 nm than those observed for corresponding F x TPP.

Light-driven CO 2 RR
The light-driven properties of these macrocyclic chromophores were evaluated in a well-studied CO 2 RR system in our laboratory, by using an iron (III) tetrakis (2',6'-dihydroxyphenyl)-porphyrin (FeTDHPP) as the catalyst and 1,3-dimethyl-2-phenyl benzimidazoline (BIH) as the electron donor 52,53 .The system containing these three components in a CO 2 -saturated DMF solution was irradiated using a red light-emitting diode (λ max = 630 or 730 nm, Supplementary Fig. 8), and the gaseous products generated were measured in real-time by gas chromatography (GC).
We first examined the activity of a simple meso-tetraphenyl porphyrin F 0 TPP for red-light-driven CO 2 RR.In a system (50 μM F 0 TPP, 1.0 μM FeTDHPP, 50 mM BIH, 630 nm light), only a small amount  (0.02 μmol) of CO corresponding to a TON of 4.0 was detected.We found that replacing F 0 TPP with F 0 Ch in the system resulted in a >52fold increase of TON at 630 nm (Fig. 2a).A remarkable observation for both F x TPP and F x Ch is the increases in CO 2 RR activity with more fluorine substituents on the chromophore (Fig. 2a-c, and Table 1).
With the per-fluorinated chlorin F 20 Ch, the system achieves a high TON of 1790 ± 52 after 51 h of irradiation, an initial turnover frequency (TOF) up to 194 ± 9 h −1 , and a quantum yield of 0.88 ± 0.03% at 630 nm (Supplementary Table 3).The TON described here is significantly higher than those reported for red-light-driven CO 2 RR (homogeneous and heterogeneous) systems including the ones using noble metals (Supplementary Tables 1 and 2).We further found that the initial rates of CO production followed a first-order dependence on both concentrations of chromophore and catalyst (Supplementary Figs.11 and 12).Based on the observation, we hypothesized that high TONs for both chromophore and catalyst could be realized in one system, which would be beneficial for the development of versatile light-driven and light-electricity-driven systems.Indeed, when the CO 2 RR experiments were conducted under the same concentration of F 20 Ch and FeTDHPP, a high TON of 1404 was obtained after 147 h (Supplementary Table 4).
In all the red-light-driven experiments performed under an atmosphere of CO 2 , no other gaseous product was observed by GC.Analysis of the liquid phase by 1 H NMR showed no detection of formic acid and methanol.To study the selectivity of the system further, we found that the amounts of CO generated were near the theoretical maximum value (Supplementary Fig. 13) of the electron donor BIH (based on two electrons per BIH molecule), which suggests that the selectivity of CO is nearly 100%.
We note that both the stability of the system and the amount of produced CO are significantly improved at higher concentrations of F 20 Ch and FeTDHPP (Fig. 2d).To study stability and scalability of the system further, we found that the photocatalysis lasted over 240 h and produced over 608 μmol CO when the experiments were conducted at 0.1 mM F 20 Ch and FeTDHPP (Fig. 2e).The slight decrease of the initial catalytic rate is presumably due to consumption of CO 2 during CO 2 RR.Indeed, we observed slower rates of CO generation from mixtures at lower concentrations of CO 2 (Fig. 2f).However, its ability to function at low CO 2 contents (down to 1%) with high selectivity (97.7% under 5% CO 2 and 95.6% under 1% CO 2 ) was impressive.
To study the nature of the system, various control experiments were performed.We observed no CO generated from a light-driven system carried out under an atmosphere of N 2, which implied that CO was derived from CO 2 .Isotopic labeling experiments conducted under 13 CO 2 produced 13 CO as detected by GC-MS (Supplementary Fig. 14).This result thus further confirms that CO 2 is the carbon source in catalysis.In addition, a negligible amount of CO detected in the absence of F x Ch, FeTDHPP, BIH, light, or under Ar (Supplementary Table 6) suggests that all components are essential for the lightdriven CO 2 RR.To rule out potential metal contaminants, inorganic salts (Fe 3+ , Cu 2+ , Ni 2+ , Co 3+ , Ru 3+ , Pd 2+ ) with or without TDHPP ligand all produced no or negligible amount of CO as compared with the experiment using FeTDHPP as the catalyst (Supplementary Table 7).Photolysis performed using chemicals that passed the elemental analysis or using FeTDHPP synthesized from highly pure FeBr 2 all showed identical activity (Supplementary Fig. 15).Furthermore, experiments conducted in the presence of an excess amount of Hg 0 showed an identical activity profile (Supplementary Fig. 16), indicating no contamination from amalgam-forming metals.Dynamic light-scattering measurements showed the absence of nanoparticles in the CO 2 RR systems before and after light irradiation (Supplementary Fig. 17).

Mechanistic study
To gain mechanistic insight into the red-light-driven system, we next sought to identify the active intermediates in CO 2 RR.Under Ar or N 2 , the cyclic voltammogram (CV) of F 0 Ch in DMF shows two reversible redox events at -1.11 and -1.58 V vs SCE (Fig. 3a, Supplementary Fig. 18, and Table 1), corresponding to the generation of one and two electrons reduced Chs (F 0 Ch -and F 0 Ch 2-) 39 .For the fluorinated Chs, these two reduction potentials shift towards more positive (by over 300 mV for F 20 Ch) due to the electron-withdrawing effects of the fluorine substituents (Fig. 3a, Supplementary Fig. 19, and Table 1).Similar CV spectra were obtained in experiments conducted in the presence of 1% H 2 O (Supplementary Fig. 20).Because CO 2 RR has to occur at an Fe(0) state of FeTDHPP at -1.55 V vs SCE [54][55][56] , a more reducing form of chromophore other than F x Ch -and F x Ch 2-must be involved in the photochemical scheme.Indeed, the CV, obtained under an atmosphere of CO 2 and in the presence of H 2 O as the proton source, revealed the appearance of a new reduction wave at a potential more negative than -1.6 V (Fig. 3b, Supplementary Fig. 21, and Table 1).A similar observation was obtained when using either trifluoro ethanol or acetic acid as the proton donor (Supplementary Figs.22-26).In these experiments, the increase of the first reduction wave and the decrease of the second reduction wave suggests the generation of F x ChPh (Fig. 4) through two subsequent proton-coupled electron transfer 47 .Therefore, the wave at <-1.6 V is ascribed to further reduction of F x ChPh to F x ChPh -(Fig.4), which is thermodynamically favorable in reducing FeTDHPP to the required Fe(0) intermediate for CO 2 RR.
We gained further evidence of the F x ChPh species using ultraviolet-visible (UV-vis) spectroscopy.By photolyzing a solution containing F 0 Ch and BIH for a few minutes, we observe a broad absorption peak (from ~450 nm to 850 nm) that corresponds to a 2e -/2H + photoproduct F 0 ChPh (Supplementary Fig. 27) 39,46,47 .Consistent with this result, similar broad spectra are observed in the reduction of other F x Ch (Supplementary Figs.28-30).In addition, most of the F 0 Ch can be recovered in a reverse 2e -/2H + oxidative process by exposing the photolyzed solution to the air (Supplementary Fig. 27c).
We next examined the UV-vis spectra of the catalytic solutions (containing F 0 Ch or F 20 Ch, FeTDHPP, and BIH) during photolysis (Supplementary Figs. 31 and 32).The broad spectra corresponding to F x ChPh were again quickly generated within minutes.Continued irradiation resulted in a slow decrease of F x ChPh during CO 2 RR, suggesting a further reduction of F x ChPh to F x ChPh -.By keeping the photolyzed F 20 Ch-containing system in the dark, we observed recovery of F 20 ChPh and generation of F 20 Ch and F 20 BC in the solution (Supplementary Fig. 33), as well as an additional 0.66 ± 0.03 equiv of CO in the headspace (Supplementary Table 8).These findings thus suggest that both photochemical conversion of F x ChPh to F x ChPh - (through reductive quenching) and electron transfer from F x ChPh -to FeTDHPP (to generate the Fe(0) intermediate) are the two key steps in CO 2 RR (Fig. 4).
We performed additional experiments to study whether F x ChPh - is also involved in red-light-harvesting in CO 2 RR.Because it exhibits absorption up to ~600 nm (Supplementary Figs. 31 and 32), the corresponding photochemical pathway in CO 2 RR should be terminated by using a light source with longer wavelengths.We found that all four F x Ch were active in producing CO over 170 h under irradiation with 730 nm LEDs (Table 1 and Supplementary Fig. 34).In the series, the system with F 20 Ch gave the highest TON CO of 510.This suggests that F x ChPh instead of F x ChPh -is responsible for absorbing red light in CO 2 RR.
To evaluate the stability of F x Ch during CO 2 RR, we quenched the photolysis by treatment of the catalytic solution first using a Co(III) dimethylglyoximate complex and then exposure to the air.For the F 0 Ch-containing system, UV-vis spectra revealed a 71% decrease of F 0 Ch and a significant increase of the F 0 BC after 75 h irradiation (Supplementary Fig. 35).In comparison, 51% of F 20 Ch was recovered and a relatively small amount of F 20 BC was observed (Supplementary Fig. 36).These results imply that cessation of CO 2 RR under these conditions may be due to complete decomposition of FeTDHPP.In fact, at a higher [FeTDHPP], the lifetime of the system is significantly prolonged (Fig. 2d, e), as described above.Furthermore, addition of FeTDHPP to a photolysis system at 51 h completely restored the activity (Supplementary Fig. 37), which confirms that deactivation of FeTDHPP is a limiting factor in the lifetime of the system.
To understand the very different activity between F x TPP and F x Ch, we examined the photochemical steps of F x TPP in CO 2 RR.For the least active chromophore F 0 TPP, no conversion to F 0 Ch was observed during photolysis (Supplementary Fig. 38).In its quenched reaction mixture, we found that most of the F 0 TPP was recovered after 4 h irradiation (Supplementary Fig. 39).However, for other fluorinated TPP, F x Ch intermediates can be observed unambiguously (Supplementary Figs.40-42).Furthermore, a significant conversion of F 20 TPP to F 20 Ch was observed (Supplementary Fig. 43), which might explain the much higher activity observed with F 20 TPP in the series.This evidence suggests that the fluorine substituents on TPP facilitate isomerization from phlorin (a 2e -/2H + reduced porphyrin, defined as F x Ph) to Ch (Fig. 4), and such transformation is an essential pathway when using F x TPP as the chromophore for red-lightdriven CO 2 RR.
Because F x BC (presumably isomerized from F x ChPh) is present in the quenched photolysis mixtures, we study its impact on CO 2 RR with an independently synthesized F 20 BC.The crystal structure of F 20 BC shows two characteristic C-C single bonds in the pyrrole ring, which exhibit similar distances compared with reported Ch and bacteriochlorins (BC) compounds (Supplementary Table 10) 57,58 .Under the same conditions, the system with F 20 BC exhibits a much lower initial TOF (60 h −1 ) than that with F 20 Ch (Supplementary Fig. 44).UV-vis study shows no detection of F 20 Ch in both the photocatalytic and the quenched solutions (Supplementary Figs.45 and 46).Hence, the irreversible isomerization from F x ChPh to F x BC (Fig. 4) during photolysis might also lead to a decrease of CO 2 RR activity when using F x Ch as the chromophores.
Previous studies showed that BIH functioned as a 2e -/1H + donor 59,60 .To generate the BI-radical (which donates the second e -), deprotonation of the BIH-radical cation by a base such as triethylamine (TEA) was found to be necessary in acetonitrile (ACN) (Supplementary Table 11).However, there are several photocatalytic studies reported in DMF without TEA or additional bases when using BIH as the electron donor 53,61,62 .To investigate this, we performed CV studies for BIH in DMF and ACN (Supplementary Figs.47-48).In contrast to the voltammograms in ACN, the CV in DMF showed appearance of a reduction wave at ~-1.6 V vs SCE, corresponding to generation of the BIradical.This result suggests that deprotonation of the BIH-radical cation is more favorable in DMF than in ACN.However, addition of TEA to the system was found to improve the activity (Supplementary Fig. 49 and Supplementary Table 11), exhibiting a TON CO of 2132 in 27 h and an initial TOF CO of 584 h −1 .In the overall reactions, BI + and OH - were produced in generation of the 2e -/2H + reduced chromophores and in CO 2 reduction (Eqs. 1 and 2).

BIH + CO
The photocatalytic CO 2 reduction mechanism by FeTDHPP has been extensively investigated by Robert and co-workers [63][64][65] .UV-vis studies showed generation of the corresponding Fe(II) and Fe(I) species at the early stage of photolysis (Supplementary Figs.50-53).The Fe(I) species was found to decrease during CO production, which indicates a catalytic cycle consistent with previous reports (Fig. 4) 52,64,65 .No electrostatic interaction was found between the chlorin and FeTDHPP by UV-vis studies (Supplementary Fig. 54), which suggests electron transfer from the chromophore to the catalyst follows an outer-sphere mechanism.
Overall, red-light-driven reduction of CO 2 was achieved using a series of synthetic porphyrin-based chromophores in precious-metalfree systems.Conversion of TPP to Ch and ChPh has been identified as an important photochemical pathway in CO 2 RR.Fluorination of the light-harvesting macrocycle has been demonstrated to be an effective method both in facilitating such transformation and in promoting the catalytic activity.In light of the high TON, long-term stability, and selectivity of the systems, we anticipate that this study maps a route for the development of efficient CO 2 RR systems using low-energy sunlight.

Preparation of Co(dmgH) 2 PyCl
Co(dmgH) 2 PyCl was synthesized following a modified procedure based on previous report 66 .CoCl 2 •6H 2 O (2.15 mmol, 500 mg) was dissolved in 200 mL ethanol and heated to 70 °C, then dimethylglyoxime (4.70 mmol, 551 mg) was added.After 10 min of stirring, pyridine (4.30 mmol, 344 mg) was added drop by drop to the mixture and air was bubbled through the solution for 30 min.The yellow precipitate was collected by filtration and washed with deionized water, ethanol, and diethyl ether, and dried under vacuum.Large yellow block crystals were obtained from acetonitrile by slow evaporation at ambient temperature (75% yield).Co(dmgH) 2 PyCl was evidenced by 1 H NMR (Supplementary Fig. 86).

Preparation of BIH
BIH was prepared based on modified methods in the literature 67 .2-Phenylbenzimidazole (30.91 mmol, 6.00 g) was dissolved in 30 mL methanol solution containing NaOH (32.00 mmol, 1.28 g), then methyl iodide (112.38 mmol, 7 mL) was added to the above solution and the mixture was heated at 100 °C for 24 h in the dark.After cooling to room temperature, the faint yellow solid (BIH + I − ) was collected by filtration and washed with EtOH/H 2 O (5:1, v/v).Then, a solution of BIH + I − (8.57mmol, 3.00 g) in methanol (80 mL) was added slowly with NaBH 4 (89.47 mmol, 3.40 g) under N 2 and the mixture was allowed to react for 3 h.The resulting solution was evaporated to obtain a white solid.The white solid (BIH) was purified by washing with plenty of water.The yield was 90%.BIH was evidenced by 1 H NMR (Supplementary Fig. 84) and elemental analysis (calcd., found for C 15 H 16 N 2 ): C (80.32, 80.20), H (7.19, 7.26), N (12.49,12.42).

Preparation of F 4 TPP
F 4 TPP was synthesized based on modified methods in the literature 69 .Pyrrole (0.05 mol, 3.355 g) and 2-fluorobenzaldehyde (0.05 mol, 6.205 g) were added dropwise simultaneously to a boiling propionic acid (200 mL) and the mixture was refluxed for another 30 min.When the resulting solution was cooled to ambient temperature, a purple product (16% yield) was obtained by filtration, and washed with methanol then dried under vacuum.F 4 TPP was evidenced by 1  Preparation of F 12 TPP and F 20 TPP F 12 TPP and F 20 TPP were synthesized according to modified methods from the literature 70 .2,4,6-Trifluorobenzaldehyde (13.00 mol, 2.080 g) or pentafuorobenzaldehyde (13.00 mol, 2.548 g) was dissolved in 500 mL dichloromethane (DCM), followed by addition of pyrrole (13.00 mmol, 905 μL).After the mixture was stirred and degassed by N 2 for 20 min, BF 3 •Et 2 O (3.90 mmol, 1.1 mL) was added via a syringe.After 2 h, TEA (7.80 mmol, 1.0 mL) was added to neutralize excessive acid, then DDQ (13.65 mmol, 3.100 g) was added and the resulting mixture was stirred for an additional 1 h.The residues were purified by column chromatography on silica gel eluted with Hex/DCM (V Hex :V DCM = 4:1).Both yields for F 12 TPP and F 20 TPP are 24%.F 12 TPP and F 20 TPP were all evidenced by 1 H NMR, 19 F NMR (Supplementary Figs.69-72)

Preparation of F x Chs
F x Chs were synthesized based on modified methods in the literature 48,49 .Note: the chlorin-based compounds F 0 Ch 48 , F 20 Ch 71 , and F 20 BC 72 have been previously reported.F 4 Ch and F 12 Ch are new compounds.
Purification procedure: After cooling to room temperature, the reaction mixture was added to 200 mL water and then extracted with DCM.The extracted organic portion was washed with 2 M HCl (3 times), saturated sodium bicarbonate aqueous solution (2 times) and deionized water (3 times).Appropriate amounts 2,3-dichloro-5,6dicyano-benzoquinone (DDQ, 1 mg/mL) in DCM were slowly added to the collected DCM layer until the characteristic absorption of the overreduced product at ~740 nm (for synthesis of F 0 Ch and F 4 Ch), ~744 nm (for synthesis of F 12 Ch), ~748 nm (for synthesis of F 20 Ch) disappeared.The solvent was removed and the crude product was purified by silica gel column chromatography using DCM (for purification of F 0 Ch, F 4 Ch, and F 12 Ch) or PE/DCM (V PE :V DCM = 50:1) (for purification of F 20 Ch) as the eluent to give the corresponding chlorin, which were characterized by 1 H NMR, and/or 13

Preparation of F 20 BC
F 20 BC was synthesized based on a modified method in the literature 72 .F 20 TPP (0.13 mmol, 125 mg,) and TSH (3.90 mmol, 740 mg) were added to a mortar, and then grinded evenly.The powder was put into a Schlenk flask and kept under vacuum for 12 h.Subsequently, the mixture was heated to 160 °C and kept for 30 min.After cooled to room temperature, the mixture was purified by silica gel column chromatography using PE/DCM (V PE :V DCM = 20:1) as the eluent and washed with Hex to obtain the corresponding F 20 BC (8% yield).Recrystallization of F 20 BC by vapor diffusion of Hex into a chloroform solution gave block green crystals suitable for X-ray diffraction analysis.F 20 BC were evidenced by 1 H NMR, 19 19 F NMR spectra were recorded on a Bruker advance III 400-MHz NMR instrument at room temperature.UV-vis spectra were acquired using a Thermo Scientific GENESYS 50 UV-visible spectrophotometer.HRMS spectra were collected on a Thermo Scientific Orbitrap Q Exactive ion trap mass spectrometer.Dynamic light scattering experiments were conducted with a Brookhaven Elite Sizer zatapotential and a particle size analyzer.C/H/N analysis for all the photosensitizers, catalyst, and electron donor were recorded on vario EL cube elemental analyzer.

Photocatalytic CO 2 reduction
A typical photocatalytic CO 2 reduction experiment was carried out in a glass vial (56.8 mL) upon successive addition of DMF solution (5 mL) containing BIH, FeTDHPP, and the chromophore.The glass vial equipped with a magnetic stirrer was sealed with an airtight rubber plug and purged with CO 2 for at least 25 min.The reaction sample was then irradiated with a red LED light setup (λ = 630 nm or 730 nm, PCX-50C, Beijing Perfectlight Technology Co., Ltd.).The gaseous products in the headspace were analyzed by Shimadzu GC-2014 gas chromatography equipped with Shimadzu Molecular Sieve 13X 80/100 3.2 × 2.1 mm × 3.0 m and Porapak N 3.2 × 2.1 mm × 2.0 m columns.A thermal conductivity detector (TCD) was used to detect H 2 and a flame ionization detector (FID) with a methanizer was used to detect CO and other hydrocarbons.Nitrogen was the carrier gas.The oven temperature was kept at 60 °C.The TCD detector and injection port were kept at 100 °C and 200 °C, respectively. 13C isotopic labeling experiments were conducted in a 13 CO 2 atmosphere and the gas products were analyzed by GC-MS (Thermo Scientific TSQ Quantum XLS).

Photolysis quenching
During photolysis, 2.52 μmol Co(dmgH) 2 PyCl in DMF (210 μL) was injected into a photocatalytic solution under N 2 and the mixture was allowed to stir for 3 h in the dark.The mixture was then exposed to the air for 1 h and analyzed by UV-vis spectroscopy.Direct exposure of the reaction mixture to the air led to complicated oxidized species with unidentified UV-vis spectra.

Electrochemical measurements
Cyclic voltammetry (CV) and square wave voltammetry (SWV) were performed on a CHI-760E electrochemical workstation, using a glassy carbon working electrode (diameter 3 mm), Pt auxiliary electrode, and a SCE reference electrode.The electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) in DMF or DMF/H 2 O.The solution was purged with N 2 or CO 2 at least 20 min before measurements.All reported potentials in this work are versus SCE.

Fluorescence and excited-state lifetime determination
A solution of the chromophore in a closed quartz cuvette with a septum cap was purged with N 2 for at least 15 min.The steady-state fluorescence was recorded on the Duetta fluorescence and absorbance spectrometer.The excited-state lifetimes (τ 0 ) of F x Ch were measured with an FLS 980 or FLS 1000 fluorescence spectrometer (Edinburgh instruments), in which a picosecond pulsed diode laser (λ = 472 nm) (Edinburgh instruments EPL470) was used as the excitation source.

Quantum yield measurement
The experiments were conducted on 630 nm LED light.The blank was a DMF solution containing 5 µM FeTDHPP, and 50 mM BIH.The difference between the power (P) of light passing through the blank and through the sample containing F x Ch (x = 0, 4, 12, 20) was measured with a FZ-A Power meter (Beijing Normal University Optical Instrument Company).The quantum yield (Φ) was calculated according to the Eq.(3): that is, where n(CO) is the number of molecules of CO produced, I is the number of incident photons; I can be calculated by the Eq. ( 5): S is the incident irradiation area (S = 6.33 cm 2 ), t is the irradiation time (in second), λ is the wavelength of the light (630 nm), h is the Plank constant (6.626 × 10 −34 J•s), and c is the speed of light propagation (3 × 10 8 m•s −1 ).

X-ray crystallography
X-ray diffraction data were collected on SuperNova single crystal diffractometer using the CuKα (1.54184 nm) radiation at 150 K. Absorption correction was carried out by a multiscan method.The crystal structure was solved by direct methods with SHELXT 73 program, and was refined by full-matrix least-square methods with SHELXL 73 program contained in the Olex2-1.5 74 .Weighted R factor (Rw) and the goodness of fit S were based on F 2 , conventional R factor (R was based on F (Supplementary Table 9).Hydrogen atoms were placed with the AFlX instructions and were refined using a riding mode.Figures were drawn with Diamond software.

Fig. 2 |
Fig. 2 | Photochemical data of CO 2 RR. a Comparing the overall TON CO of systems with different F x TPP and F x Ch. b TON CO for systems with different F x TPP.c TON CO for systems with different F x Ch. d CO generation for systems with different initial [F 20 Ch] and [FeTDHPP].e stability of a system with F 20 Ch, f CO 2 RR under 1% or 5% concentrations of CO 2 with F 20 Ch.Error bars denote standard deviations.TON calculated based on [FeTDHPP].Catalytic conditions: a-c used 50 μM F x TPP or F x Ch, 1.0 μM FeTDHPP, and 50 mM BIH; d used the same concentrations (1, 2, 5,

Fig. 3 |
Fig. 3 | Electrochemical data.Cyclic voltammograms of 0.25 mM F 0 Ch, 0.5 mM F 4 Ch, 0.5 mM F 12 Ch, and 0.5 mM F 20 Ch in DMF containing 0.1 M TBAPF 6 at scan rate 0.1 V/ s. a Under N 2 .b Under CO 2 with 1% H 2 O. Source data are provided as a Source data file.